1. Field of the Invention
The invention concerns an ATM switching matrix including a passive optical core.
2. Description of the Prior Art
The drawback of prior art matrices of this type is that they include buffers in the form of sections of optical fiber for storing cells, exploiting the fact that signals take a certain time to propagate through them. These optical fiber sections are bulky. They therefore constitute a physical limit on the implementation of such matrices.
The aim of the invention is to remedy this drawback.
The invention consists in an ATM switching matrix having n input ports, a passive optical core and n output ports each having an electronic buffer, wherein:
each input port includes:
an input coupled to an input of the matrix,
at least p outputs, p being less than or equal to n,
a tunable light source for emitting an optical signal transporting cells received at an input of the matrix,
switching means for routing the optical signal emitted by the source to at least one of the p outputs of the input port,
the matrix further includes tuning means for tuning the tunable light source of each input port so that its wavelength corresponds to the output of the matrix which is the destination of the cell currently being received by the input port concerned,
the passive optical core includes at least p optical couplers each having 2n ports, n of which are respectively connected to an output of each of the n input ports,
each output port includes:
at least p optical receivers which are tuned to the same fixed wavelength which is specific to the output port, the receivers each having an input respectively connected to a port of one of the p couplers,
a buffer having p inputs respectively connected to the outputs of the p receivers and an output connected to an output of the matrix, and
the matrix includes distributed allocation means adapted to control the switching means of each input port to connect, to k separate couplers, k respective input ports which are receiving at the time concerned k cells addressed to the same output port, k being less than or equal to p, this operation being repeated for each output port.
The above matrix avoids the use of optical delay lines because cells are stored at the output ports in electronic buffers, combined with efficient routing by optical means coupling the input ports to the output ports. These optical means comprise optical switches, tunable lasers, filters and passive couplers, provide high transmission speeds and do not alter the sequencing of the cells. Using p passive couplers enables up to p cells to be transmitted to the same output port during the same cell period. The cell loss rate due to contention can be reduced as much as may be required by increasing the value of p.
In one preferred embodiment, the allocation means include:
in each input port, request means for sending a coupler allocation request message each time that a cell is received by the input port, the message being sent at the wavelength specific to the output port which is the destination of the cell and including the identifier of the input cell,
in each output port, means for receiving a coupler allocation request message sent at a wavelength specific to the output port and containing the identifier of at least one requesting input port, means for deciding whether or not to allocate a coupler, and means for sending a response message containing the identifier of the allocated coupler and addressed to the requesting port, and
in each input port, means for receiving a response message addressed to the input port and for controlling the switching means of the input port so as to connect it to the coupler allocated to it.
The above matrix offers high performance in terms of switching speed, even with a large number of inputs and outputs, because the coupler allocation means are distributed between all the output ports and all the input ports of the matrix. They therefore operate independently for each output port and address contention problems in a decentralized manner. There is therefore a saving in control time, which is significant when the matrix is large.
The invention will be better understood and other features of the invention will become apparent from the following description and the accompanying drawings.